1. Field of the Invention
The present invention is directed to a coupler for coupling light to a multi-mode fiber.
2. Description of Related Art
It is very difficult to manufacture a multi-mode fiber with good control over the index of refraction in the center of the fiber. If the light coupled to the fiber excites some modes that propagate mostly in the center of the fiber and other modes which do not propagate mostly in the center of the fiber, very different propagation times for these modes may result. This is referred to as differential mode delay. Differential mode delay tends to spread out the pulse length of signals and reduce the effective bandwidth of the fiber.
Modes which propagate mostly in the center of the fiber are the lower order fiber modes, i.e., modes having small propagation angles that strike at or near the center of the fiber. These lower order modes spend most of the time in the center of the fiber, tend to travel straight down the fiber and the shape of these modes does not change much as they propagate. Therefore, in order to reduce differential mode delay, any light which enters near the center of the fiber needs to be incident at an angle which is large enough not to excite lower order modes, but not so large that the critical angle is exceeded and the light fails to be coupled or no light should be input to the center of the fiber.
One current solution involves coupling light into single mode fibers which are then positioned off-axis relative to the multi-mode fiber. Single mode fibers have a much smaller core than multi-mode fibers, so can be used to provide light at specific positions on the endface of the multi-mode fiber. However, single mode coupling is more expensive than multi-mode coupling and the additional coupling step leads to an increase loss in light. Further, while no light enters the fiber of the center for this configuration, the light will still cross the fiber axis as it propagates, thus increasing the differential mode delay. Additionally, ferrules or other structures housing the multi-mode fiber:to a single mode fiber junction are not readily available and must be developed specifically for that purpose.
Another solution is to use a vertical cavity surface emitting laser (VCSEL) excited to radiate in a ring mode. The operation of the VCSEL in radiation modes other than the lowest order have less predictable flux distributions than in the lowest order mode, in which the distribution more closely approximates a Gaussian profile. Further, there will still be some power in the lower order modes of the VCSEL. Additionally, such operation of the VCSEL often requires a higher current to drive the source into the higher radiation modes.
The present invention is therefore directed to a multi-mode fiber coupler which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
More specifically, it is an object of the present invention to reduce differential mode delay while coupling light into a multi-mode fiber.
It is another object of the present invention to efficiently couple light into a multi-mode fiber without requiring an offset fiber or additional beam shapers for the light source.
It is another object of the present invention to use an optical element to direct light away from the center of the fiber.
It is a further object of the present invention to provide coupling to a multi-mode fiber which is relatively insensitive to variation in light output from a light source.
It is yet another object of the present invention to provide a diffractive element which generates a beam propagating in a spiral fashion.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation.